1. Technical Field of Invention
This invention relates to a technique for controlling induced dynamic instability in an axially moving form, and, in particular, to a method and apparatus for effecting such control.
2. Background of the Art
In various industrial processes, including the manufacture of pipes, filaments and films, it may be desirable, or necessary, to expose a relatively rigid axially moving form to a treatment which will weaken the form and induce instability therein. For example, longitudinal orientation of a polymeric filament may be effected by feeding the filament at a controlled rate to an orientation zone in which the filament is softened by heating, and withdrawing the softened filament at an increased rate. Likewise, an oriented tubular film is usually produced by extruding a relatively thick-walled and rigid thermoplastics tube from an annular orifice, and subsequently stretching the extruded tube, at a temperature above the glass transition temperature (Tg) and below the melting temperature of the thermoplastics material, in the transverse and/or longitudinal directions to form a thin film, the stretching being effected in the transverse direction by means of internal gas pressure and in the longitudinal direction by withdrawing the tube at an appropriate rate in the direction of extrusion.
Temperature control during the heat treatment is of paramount importance and problems are generally encountered because of an apparent inability to achieve uniform heating around the form. While an extruded polymeric form is at relatively high temperature, the viscosity and tensile strength thereof are low and there is a tendency for the form to be positionally unstable and to wander randomly from side to side. Not only is the form liable to rupture while in this unstable condition, but the instability may lead to marked variations and irregularities in the thickness profile of oriented tubular film produced therefrom--rendering the resultant film commercially unacceptable.
Heating of an axially moving form to the desired treatment temperature is conveniently effected by an assembly comprising a plurality of infra-red heating elements disposed in a cylindrical array surrounding the form, and individual elements may be independently controlled in an attempt to overcome circumferential fluctuations in the thermal load of the heating assembly. Nevertheless, the problem persists that a significant proportion of commercially unacceptable film is produced, possibly as a consequence of external influences--such as exposure of the heated form to adventitious air currents in the vicinity of the production unit.
3. The Prior Art
Many attempts have been made to provide a solution to the problem of positional instability in polymeric tubular extrudates and of non-uniform wall thickness in films formed therefrom. For example, British patent GB No. 1440651 discloses a technique for controlling the cooling of a freshly extruded tube of a thermoplastics material, which may subsequently be stretched to yield an oriented tubular film, by feeding the tube, in the direction of extrusion, in heat-transfer relationship with an adjacent cooling surface, maintaining the cooling surface at a temperature below the melting point of the thermoplastics material, maintaining a sheath of a heat-transfer fluid between, and in contact with, the tube and the cooling surface, and cooling at least a selected area of the extruded tube by displacing the tube transversely of the direction of extrusion to decrease the thickness of the fluid sheath between the selected area and the cooling surface. In the production of tubular film the cooling surface is suitably a cooled, tapered mandrel located within the tube, and transverse displacement of the tube may be effected by a ring assembly comprising a plurality of rollers surrounding the tube.
British patent GB No. 1404947 discloses a technique for stabilising the position of a tubular film by passing a tubular film through an annular sleeve having a gas-permeable, convex arcuate surface disposed towards the film, and supplying gaseous fluid inwardly through the arcuate surface to create a cushion of gas between the film and arcuate surface to support the film out of contact with the surface.
British patent GB No. 2074349 discloses a technique for controlling the thickness of a blown tubular film in which film sectors of equal cross-sectional areas are allocated to corresponding tempering sectors of equal circumferential length on the die nozzle ring and the relevant tempering sectors of the nozzle ring are cooled to produce film in which the corresponding sectors are of the desired thickness or heated to produce film in which the corresponding sectors are of the desired thinness.
Despite these various proposals to control the cooling of a cast extrudate, to stabilise the position of a film bubble, and to effect differential temperature control of an annular extrusion die, the problem of positional instability in the production of tubular extrudates has not been overcome to the extent necessary to satisfy the increasingly stringent product quality standards demanded in the market place.
We have now devised an improved control technique.